1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus capable of displaying one of a two-dimensional image and a three-dimensional image partially when the other of the two-dimensional image and the three-dimensional image is being displayed.
2. Related Art
A method of recording a stereoscopic image by using some method and reproducing it as a stereoscopic image is known. This method is called integral photography (hereafter referred to as IP method) for displaying a number of parallax images or light ray reproduction method. It is supposed that an object is viewed with left and right eyes. When a point A located at a short distance is viewed, an angle formed by the point A and the left and right eyes is denoted by α. When a point B located at a long distance is viewed, an angle formed by the point B and the left and right eyes is denoted by β. The angles α and β vary depending upon the position relation between the object and the viewer. The difference (α−β) is called binocular parallax. Human being is sensitive to the binocular parallax and is able to conduct stereoscopic viewing.
In recent years, development of stereoscopic image display apparatuses without glasses has been promoted. Many of them use the ordinary two-dimensional plane display device. Many of them are made possible by placing some optical plate on the front or back of the plane display device, utilizing the binocular parallax described above, and controlling angles of light rays from the plane display device so as to cause light rays to appear to be illuminated from objects located several cm before and behind the plane display device when viewed by a viewer. This is because it has become possible to obtain an image which is high in definition to some degree even if light rays of the plane display device are distributed to several kinds of angles (called parallaxes), owing to implementation of the plane display device having a higher definition.
A three-dimensional (hereafter referred to as 3D as well) display method implemented by thus applying the IP method to a stereoscopic image display device is called integral imaging (II) scheme. In the II scheme, the number of light rays illuminated from one lens corresponds to the number of element image groups. The number of the element image groups is typically called number of parallaxes. In each lens, parallax rays are illuminated in parallel. In the II scheme, the viewer views different images: an image of 1 parallax, an image of 2 parallaxes, and an image of 3 parallaxes, according to the position of the viewer or the angle at which the viewer views.
Therefore, the viewer perceives a solid body by parallax between the right eye and the left eye. If a lenticular lens is used as the optical plate, there is a merit that the display is bright because the utilization efficiency of light is high as compared with a slit. As for a gap between the lens array and pixels, it is desirable to provide a distance which is nearly equal to the focal length. By doing so, one pixel can be illuminated in one direction and parallax images which differ according to the viewing angle can be viewed.
The best-known substance having a double refraction property is calcite. As an optical application of the double refraction, there is a drawn film used for the retardation film. Specifically, ARTON (JSR Corporation) and polycarbonate (Nitto Denko Corporation) are well known.
Furthermore, liquid crystal also has the double refraction property. In the liquid crystal, each molecule takes a long and slender shape. Anisotropy of the refractive index occurs in a lengthwise direction of the molecule called director. For example, many of molecules in nematic liquid crystal are long and slender molecules. Their major axis directions are aligned and oriented. However, position relations of the molecules are random. Even if the orientation directions of molecules are in alignment, the absolute temperature of ambience in use is not zero degree, and consequently they are not perfectly parallel and there is fluctuation (represented by an order parameter S) to some degree. Viewing a local region, however, it can be said that molecules are aligned in nearly one direction. When a region which is small enough macroscopically but large enough as compared with the size of the liquid crystal molecules is supposed, the average orientation direction of the molecules in that region is represented by using a unit vector n, and it is referred to as director or orientation vector. An orientation in which the director becomes nearly parallel to the substrate is referred to as homogeneous orientation. One of the greatest features of liquid crystal is optical anisotropy. Especially, since the degree of freedom in the molecule arrangement is high as compared with other anisotropic media such as crystal, the difference in refractive index between the major axis and the minor axis which is a criterion of double refraction is great.
In simple matrix drive which is one of drive schemes for driving the liquid crystal, a configuration obtained by interposing a liquid crystal layer between row electrodes Xn arranged in one column and column electrodes Ym is used. The liquid crystal is activated by selectively applying a voltage to a part (pixels) where these electrodes intersect. In this drive method, electrodes are formed of transparent electrodes and a transistor which individually controls ON-OFF is not provided in each pixel. Therefore, there are no black matrixes for hiding wires. This results in a merit that the luminance can be made bright.
In active matrix drive which is another drive scheme for driving the liquid crystal, the electro-optic effect itself of the liquid crystal is provided with a memory property. For example, if a voltage is applied stationarily over a frame period once changeover from the ON state to the OFF state is performed, then high definition display becomes possible in principle even when the display capacity is increased remarkably. As an element having such a memory property, there is an active element such as a transistor or a diode. In the configuration according to the active matrix scheme, the active element is added to each pixel. A merit of this active matrix scheme is that polarization changeover which is free from the concern about crosstalk, which is high in definition and which does not depend upon the wavelength is possible (see “The foundation of liquid crystal and display application” written by Y. Yoshino and M. Ozaki published by CORONA PUBLISHING CO., LTD.)
A method for suppressing the frame response is reducing the potential difference between a selection pulse and a non-selection pulse in the simple matrix drive. As a method for shortening the selection pulse interval without making the pulse width small, there is a multi-line selection (MLS). According to the MLS, a plurality of scanning lines is selected simultaneously unlike the conventional line sequential scanning (see Y. Kaneko, et al. “Full Color STN Video LCDs,” Eurodisplay '90 Digest, p. 100, 1990).
In the stereoscopic image display apparatus, image information of the plane display device disposed on the back of the optical plate is assigned to respective parallax images. Therefore, the resolution falls as compared with the original plane display device disposed on the back. Therefore, a function capable of changing over between high definition two-dimensional image display and three-dimensional image display providing a stereoscopic sense in the same stereoscopic display apparatus is desired. In addition, there is also a strong demand for a function of providing the high definition two-dimensional image display and the three-dimensional image display on the same display plane, i.e., a function capable of changing over between a two-dimensional image and a three-dimensional image partially. For implementing changeover between the two-dimensional image and the three-dimensional image partially, it can provide variable polarization cells, divide electrodes for applying a voltage to liquid crystal of the variable polarization cells by X coordinates and Y coordinates, and apply individual voltages to respective areas.
Drive methods for variable polarization cells can be broadly classified into the following three kinds.
(1) Segment drive
(2) Simple matrix drive
(3) Active matrix drive
The segment drive in (1) is a drive method frequently used in watches and electronic calculators. Individual display parts (optical switches) are formed of independent electrodes. If matrix drive is conducted, display disturbance is caused in the display part by wiring. Therefore, the segment drive in (1) is not suitable for the drive method of conducting the changeover between the two-dimensional image and the three-dimensional image partially.
In the simple matrix drive in (2), matrix drive can be conducted as described above. However, signals input to the same row and the same column are applied in the same way. Therefore, it is necessary to apply a voltage of at least a threshold to a pixel only when the pixel is selected and suppress the voltage to the threshold or below when the pixel is not selected. As the resolution (i.e., the number of address lines in the vertical direction) of the variable polarization cells becomes large, the ratio between a voltage applied to liquid crystal when the pixel is selected and that when the pixel is not selected approaches unity and consequently the selection ratio between the two-dimensional image display mode and the three-dimensional image display mode becomes small. A concrete degradation phenomenon is that noise is mixed in the far-side or near-side display at the time of a three-dimensional image display mode because a two-dimensional image display mode is mixed and the directivity of light rays is degraded. If Twisted Nematic (TN) liquid crystal obtained by twisting nematic liquid crystal by 90 degrees is used in the simple matrix drive, then the threshold characteristics cannot be made sufficiently steep, resulting in a problem that the number of addresses is limited.
If Super Twisted Nematic (STN) having a twist angle of approximately 270 degrees and steep threshold characteristics is used instead of TN liquid crystal, therefore, the contrast can be maintained even when the voltage ratio approaches unity. Since the STN liquid crystal has great dependence of polarization characteristics upon the wavelength, however, it is necessary to use a film which compensates the wavelength dependence at the time of use and complicated optical design and a retardation film material become necessary.
In the active matrix drive in (3), each pixel is driven individually in one frame section as described above. Therefore, it is possible to use TN liquid crystal. As demerits, however, it can be mentioned that the aperture ratio caused by the black matrix falls (i.e. the luminance falls) and moire is caused by interference between a black matrix of variable polarization cells and a black matrix of an LCD for display pixel. Furthermore, complication of the manufacture process and a cost increase caused by the manufacture process of the Thin Film Transistor (TFT) can be mentioned.